Break dependency of init on wallet.

This required some code movement (what was CWalletTx::AcceptToMemoryPool
doing in main?), and adding a few explicit includes that used to be
implicit through init.h.

src/qt/optionsmodel.cpp

@@ -6,6 +6,9 @@
 
 #include "bitcoinunits.h"
 #include "init.h"
+#include "core.h"
+#include "wallet.h"
+#include "netbase.h"
 #include "walletdb.h"
 #include "guiutil.h"